Electronic gas control circuit



Aug. 22, 1950 T. B. PERKINS ELECTRONIC GAS CONTROL CIRCUIT Filed June 19, 1946 menmm/ Maw/V ATTORNEY Patented Aug. 22, 1950 ELECTRONIC GAS CONTROL CIRCUIT Theodore B. Perkins, West Orange, N. J., minor to Radio Corporation of America,

of Delaware a corporation Application June 19, 1946, Serial No. 677,875 4 Claims. (Cl. 236-21) My present invention relates to electronic gas control circuits, and more particularly to an improved electronic control circuit for a gas heating system.

An important object of my invention is to provide a control circuit including a grid-controlled alternating current rectifier device responsive to extinction of a pilot light for shutting off the gas supply valve of a gas burner line.

Another important object of this invention is to provide a thyratron tube whose cathode and anode cooperate to function as a rectifier of alternating current to maintain a gas supply valve open, and the control grid of the thyratron being included in a circuit which is closed by a pilot flame whereby extinction of the flame results in prevention of rectification and resultant closing of the gas supply valve.

Another object of this invention is to provide in a control system 01' the type described in the preceding paragraph, a resistive element responsive to excess heating temperature of the hot water chamber for increasing the voltage of the control grid of the thyratron in a, negative sense thereby to cause the gas supply valve to close.

A more specific object Of my invention is to provide in an electronic control system for a gas supply valve, a device for preventing interference with the valve by the electronic control system during intermittent interruptions in conductivity of a circuit including the pilot light path.

Still oher objects of my invention are to improve the eificiency and reliability of gas heating control circuits, and more specifically to provide a thyratron control circuit which is economically manufactured and assembled.

Still other features and objects of my invention will best be understood by reference to the following description, taken in connection with the drawing, in which I have indicated diagrammatically a circuit organization whereby my in vention may be carried into eiiect.

Referring now to the accompanying drawing, wherein is shown the electronic control circuit employed in connection with any suitable gas heating system, I have depicted in purely schematic form those portions of the gas heater system which are required by a proper understanding of this invention. The numeral l designates a gas burner per se, or line feeding the burner, which may be located under the Water tank if the system is employed for water heating. However, if the system is a furnace, the line I may be that feeding the burner in a furnace. The

2 gas main 2 includes in its interior any suitable form of gas supply valve 3 of known construction. The valve 3 has a suitable stem 4 located within an electromagnet relay coil or winding 5 so that energization of coil 5 will cause the valve 3 to be lifted from its seat 3'. It will be understood that valve 3 may be normally biased to seating position whereby as long as coil 5 is energized the valve 3 is pulled upward against the biasing means, as for example, a simple coil spring. Of course, valve 3 may be a simple gravity device which is unseated by stem 4 being pulled upward due to energization of coil 5.

The numeral 6 designates the pilot light of the system. Only the upper, or burner, end of the tube is shown, and flame I is shown in conductive contact with the electrode or rod 8. The electrode 8 need not be in direct contact with the flame, but may be in the path of the hot gases rising from flame 1. It is essential that electrode 8 be conductively or electrically connected to metallic tube 6 by the flame or its hot gases. Of course, electrode 8 wil be made of a suitable metallic heat-resistant material. It is desired to cause valve 3 to move into its closed, or seated, position in response to the flame 1 being extinguished. If the pilot light flame were extinguished and no precaution taken to close the supply of gas to burner I, then upon the heat control device, usually a bi-metallic strip, calling for more heat there would occur a flow of gas through main 2 to burner i which would not be ignited by the pilot light. Hence, by providing the valve in the supply line to burner l and controlling its position in response to the presence or absence of pilot flame 1, the users of a gas system are guarded against accidents caused by extinction of the pilot light.

The control system uses the thyraton tube 8, which may be for example a grid-controlled gas filled tube of the 2050 or 2D21 types. The thyratron tube is well kn0wn,and includes an electron emitter or cathode I 0 which is adapted to be heated by heater element H, the heating circuit not being shown. The plate or anode H has interposed between it and cathode ID a control grid 12, which is shielded by shield grid I3 connected to cathode Hi. The black dot within the tube envelope denotes a gas filling. Briefly, the thyratron tube operates in a manner analogous to a switch, or relay, except that very little power is taken to operate it. For a certain combination of positive plate and negative control grid the plate to cathode path within the tube beqqms onductive due to ionization of gas with- 3 in the tube, 1. e., the tube "fires." For values less positive on the plate, or more negative on the control grid, the tube will not "fire or become conductive. However, once the tube becomes conductive, the control grid l2 has essen-, tially no effect on tube operation, and the plate voltage must be reduced below the tube drop itself. usually around 8 to volts, in order to extinguish the tube. In the circuit shown the control grid I2 never becomes positive.

operating value, and assures that the tube 9 is not overloaded. There may be shunted across the winding 5 a series resistor-condenser network I8 to maintain more constant flow of current through the relay winding to prevent chattering. A relay without slow-release features may tend to release on the negative cycle to the thyratron plate, when no current flows in the thyratron. The cendenser supplies current during this half-cycle. During the flow of anode current (during the positive half cycle) this condenser charges up and when the thyratron is not firing, (during the negative half cycle) this condenser will discharge into the relay to maintain its holding power. It, also, serves to damp out the transients causedby sudden changes in current through the inductance of the relay, by preventing these sudden changes. The resistance is used to prevent sudden changes in the capacitative currents.

The metallic pilot tube 6 is connected by lead I9 and slider 20to any desired predetermined point on resistor 2|. The resistor 2| is connected in series with so-called temperature coil 22 between the cathode lead and the lower end of grid leak resistor 23. The resistor 23 is connected from lead 24, which connects electrode 8 to control grid I2, to the lead 25 which is adapted to be connected to the negative terminal of a suitable source of direct current. In other words, 7

there is provided a source of direct current voltage to bias grid |2 negatively relative to cathode III. The cathode lead I1 is connected to the positive terminal of the grid biasing voltage source. A voltage regulator tube 21, say a gas tube of the VR 105 type, is shown with its anode connected to the cathode lead Ill, while its cathode is connected to the upper end of resistor 28 in lead 25. The regulator tube 21 is utilized to maintain a constant bias condition in spite of linevoltage changes. Thyratrons such as the 2050 and 2D21 are effectively high mu tubes, and, therefore, changes in plate voltage have relatively little effeet on the tubes operation. Resistor 26 may be inserted in either the anode or cathode side of "o temperature. To avoid intermittent action some appreciable difference in "on" and "oil" temperatures is desirable. Resistor III and condenser 3| are utilized to provide both a differential between firing and de-firing potentials, and a delay in signal operation. The bias control 20 sets the signal potential change required to fire the tube. Without the circuit comprising 30 and 3|, this same signal potential which causes the tube to fire would also be the potential which causes the tube to defire, which, of course, creates instability and unwarranted operation and deoperation of the relay. To overcome this difllculty, the values of 30 and 3| are so chosen as to give good stability without decreasing the sensitivity of temperature control appreciably. Since the use of these components change the signal voltage at which the thyratron will de-fire, the variable resistance 30 becomes an ofi control. Since the circuit can be made to ofier a low impedance to sudden changes in signal voltage but high impedance to low frequencies, it is capable of eliminating the eflect of sudden changes in signal voltage.

In consideraing the operation of the control circuit, it is first pointed out that the cycle alternating current is applied between 'the plate I I and cathode III of the thyratron tube 9, and, of course, the plate II will be positive only during the alternate positive half cycle of the energizing altemating current. The slider 20 is set at a point on resistor 2| such that a predetermined ngative bias is applied to grid. As long as the flame I is burning the thyratron 9 will always conduct during the positive cycle of the A. C. supply voltage to the plate II.-

In other words, the potentiometer 20, 2| sets the negative grid bias at which tube 9 will fire and keep the winding 5 energized thereby to maintain the valve 3 in open position. Upon the flame becoming extinguished the shunt circuit around 23 through 24, 8, I, 6,", 20, 2|, is broken, and the grid I2 is then connected through resistor 23 to the negative lead 25. Accordingly a much higher negative bias now exists on grid I2, and instantaneously upon the plate II assuming a negative potential on the negative half cycle of the energizing 60 cycle wave, the tube 9 will be rendered inoperative and will not again conduct on the positive half cycle. As-a result the winding 5 will be de-energized, and the valve 3 will be released thereby to close the flow of gas from the supply source to the gas line I.

It will, therefore be seen that the grid circuit 55 through the flame I actually provides a conducthe regulator tube 21, and is necessary in order to V obtain regulation.

There is provided between grid I2 and cathode III a series-path consisting of resistor 30 and condenser SL The magnitude of resistor 30 may be adjusted so as to provide a predetermined degree of delay action. The specific function of the tive path which short-circuits the highly negative bias path through resistor 23. Intermittent action, as upon the flame I wavering or being blown to and fro by a breeze, will not cause the thyratron 9 to cease firing, because the circuit 30, 3I provides a suficient delay to prevent the grid I2 from becoming highly negative unless the grid circuit through flame 1 remains broken in excess of a predetermined period of time. In other words, the resistor 30 and condenser 3| are chosen to'have a time constant such as to prevent rendering the thyratron tube inoperative in response to intermittent interference with the conduction through the flame l.

The broken line rectangle 4|! denotes a shell. or tube, housing coil 22, the tube 40 being adapted for location in the hot water chamber of the water heater system, or in a suitable location shunt path 30, II is to obtain control on the :5 of the furnace where the gas heater is used for heating. The coil 22 is a temperature-responsive element, and serves to control the grid bias of tube 9 thereby to establish a control over tube 9 in accordance with the temperature of the fiuid heated by the system. The coil 22 has a resist- When the resistance of element 22 increases due to temperature increase, there is caused to exist an increased negative bias on grid l2. Finally, the negative bias on grid l2 rises to the point that causes the thyratron to be rendered nonconductive as the current applied to plate H goes through its negative half cycle. Hence, the temperature-responsive resistor 22 provides an automatic means for increasing the negative bias on grid l2 through the pilot light circuit. If the fluid being heated calls for no more gas to be fed to burner line I, then resistive element 22 causes grid 12 to cut off tube 9 thereby to deenergize winding 5 and release valve 3. The variable resistance of 22 with temperature causes a similar eflect as moving slider 20 along resistor 2|. It varies the resistance of one arm of the bleeder, and, therefore, changes the potential of a tap between two arms of the bleeder. Element 22 usually has no apreciable inductance, although it is usually wound in coil form.

The condenser 3| is charged when the thyratron tube fires, and the condenser thus provides a maximum flow of current through winding 5. Condenser 3| is charged whenever the thyratron fires. A definite potential exists across the grid to cathode during conduction, and the condenser 3| tries to charge to this value. Whether it does or not depends on the size of the capacitor, the value of the resistor, the amount of grid current flow andthe time of conduction. It is desirable to have the magnitude of grid leak resistor 23 high compared with the conductivity of the path through the pilot flame. Resistor 2| and its slider 20 set the negative bias at which the thyratron tube 9' will fire (with the pilot flame on) and cause the gas valve 3 sired, the resistor 2| can be calibrated, in conjunction with temperature-responsive element 22, in values of temperature. Resistor 26 should have its resistance value chosen to obtain a normal value of current through regulator tube 21 for maximum regulation. The direct current supply voltage should be sum clently over the starting potential of the voltage regulator tube to give good operation.

While I have indicated and described a system for carrying my invention into eifect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention.

What I claim is:

adequate safety factor for 1. In a gas heating system including a gas conto open. If deapplying said potential to the thyratron, said biasing circuit being normally closed through a conductive path which includes the hot flame of a pilot light, means for applying a, high negative bias to the thyratron control grid in response to the pilot flame being extinguished, said means comprising a source of negative D. C. potential and circuit means for said control grid, and temperature-responsive means in circuit with said grid biasing circuit for applying a high negative bias to the control grid upon the medium being heated attaining a predetermined temperature.

2. In combination with a thyratron tube including at least a cathode, a control grid and an anode, a source of alternating current connected between said cathode and anode, said source being of sufficient magnitude to normally render said thyratron conducting, a load device responsive to rectified alternating current in circuit with said anode, a first biasing circuit connected between said control grid and said cathode, said first biasing circuit comprising a circuit path having a predetermined low resistance and including a hot gas fiame, and providing a sufficiently low negative bias to said control grid to permit said thyratron to conduct when said flame is present, a second biasing circuit connected to said control grid, said second biasing circuit including a source of negative D. C. potential and a circuit path having a resistance higher than that of said first mentioned circuit path, between said source and said grid, and adapted to apply a sufficiently high negative bias to the control grid of the thyratron, when said flame is not present, to prevent said thyratron from conducting.

3. A device according to claim 2 wherein said first biasing circuit includes a temperature-sensitive resistance element, said resistance element being responsive to a predetermined high temperature thereby to increase the negative bias of said control grid so as to prevent said thyratron from conducting.

4. A device according to claim 2 including means connected between said control grid and said cathode for delaying for a predetermined period of time the application of a high negative bias to said control grid by said second biasing circuit when the conductive path through said gas flame is broken.

THEODORE B. PERKINS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,809,280 Knowles June 9, 1931 2,112,736 Cockrell Mar. 29, 1938 2,224,119 Harrison Dec. 3, 1940 2,245,730 Sparrow June 17, 1941 2,276,803 Suits Mar. 17, 1942 2,288,890 Denison July 7, 1942 2,290,091 Brown et al July 14, 1942 2,299,501 Schneider Oct. 20, 1942 2,313,943 Jones Mar. 16, 1948 2,324,330 Smith July 13, 1943 2,343,001 Cohen Feb. 29, 1944 2,352,240 Wolfner June 27, 1944 2,360,532 Yates Oct. 17, 1944 2,410,524 Richardson et al. Nov. 5, 1946 2,431,158 Yates Nov. 18, 1947 

